Investigating the off-campus travel mode choice behavior of college students is crucial for enhancing the off-campus travel environment of college students, increasing their social participation, and further promoting their physical and mental health development. However, previous studies neglect the influence of peer groups on individual travel mode choice and lack a unified standard for identifying effective peer groups among college students. To explore the impact of peer effect on off-campus travel mode choice behavior of college students, this study defined effective groups using a roommate relationship quality scale and collected college students’ off-campus travel preference data under different travel scenarios based on questionnaire survey, identifying effective groups from a dorm social network perspective. Multiple network economics linear models were constructed by introducing a peer matrix adjusted for relationship strength, grounded in relationship quality and psychological traits, potential endogeneity issues were addressed though combining fixed effect and two-stage least squares, and the best model to identify and measure the peer effect was determined through model evaluation. Based on the model calibration results, the influence and potential mechanism of dormitory groups on individual off-campus travel mode choice of college students were analyzed. The results show that ① the local average model that incorporates peer relationship strength has better robustness in identifying and measuring peer effect compared with generalized econometric model and local aggregate model. ② With the increase of travel distance and the relaxation of travel time constraints, the influence of endogenous peer effect on individual travel mode choice behavior decreases. ③ The factors such as monthly living expenses, number of family members, driver's license, openness to experience and environmental awareness related to peers have significant exogenous peer effect on individual travel mode choice for college students. ④ The endogenous peer effect of conformity is more pronounced for extrovert individuals compared to introvert individuals. In leisure travel scenarios, the endogenous peer effect of conformity is also significantly stronger for individuals scoring high on openness to experience relative to their conservative counterparts. The findings expand the theoretical boundaries of peer influence in travel behavior and provide empirical evidence for refined guidance and subgroup-targeted interventions to promote low-carbon travel among college students.